1. Field Of The Invention:
The present invention relates to medical prosthetic implant devices and more particularly to an acetabular cup component to be fitted to a patient's acetabulum. Even more particularly, the present invention relates to a cementless acetabular cup component to be used in total hip replacement wherein there is provided an improved outer surface geometry that affixes the cup to the acetabulum with a plurality of annular, spaced apart rings, allowing a cementless press fit of the cup to the acetabulum.
2. General Background:
Total hip replacement has become a standard operation in orthopaedics for the relief of pain and restoration of function in patients who are incapacitated from disease or injury. Early total hip replacement implants were affixed to bone by means of bone cement. With time and experience, it was learned that bone cement has a finite lifetime and may fail by cracking, especially if in service for many years.
Cement failure leads to implant loosening, pain and loss of function, requiring operative intervention for correction. In the early 1980's, cementless fixation of total hip replacement implants was proposed as an alternative to cemented total hip replacement implants, on the basis that elimination of the weak link in fixation would increase the service life of total hip replacement implants.
Prior art approaches to cementless acetabular replacement include threaded components which are affixed to the acetabulum by means of screw threads. Threaded components were originally conical in shape; however, because of difficulty in placement, more recent designs have a more spherical shape. The threads of these cups are sufficiently deep to require screwing the cup into a prepared cavity for insertion. The primary disadvantage of threaded cups is the difficulty of insertion, which can lead to technical errors and poor results.
Another approach to this problem has been the spherical cup with a porous metal coating on the fixation surfaces. These cups have been affixed with a variety of fasteners, including screws, spikes and interference fits.
Screw and spike fixation have been shown to be effective; however, there are inherent risks associated with these fasteners. Surgical errors in the placement of screws have led to injuries of vital blood vessels proximate to the acetabulum, sometimes leading to the deaths of patients.
A longer term risk associated with these fixation means is the potential for fretting between the spikes/screws and the metal cup. Fretting will generate particulate metal debris which may lead to irritation and loosening of the cup, or malignant tissue responsive in the patient.
Interference fits of spherical cups are achieved by preparing the acetabulum with a diameter slightly smaller than the outer diameter of the cup which will be used. The cup is then impacted into the acetabulum during which the cavity expands to receive the cup. The expansion is partly elastic and results in a residual hoop stress in the bone at the cup/bone interface. The hoop stress applies pressure to the cup which is sufficiently large to retain it in position.
The primary disadvantage associated with spherical cups that are fit by interference is that there is very little mechanical interlock between the cup and bone. The small amount of mechanical interlock in many cases may not be sufficient to resist forces transmitted through the cup, especially if the quality of the surrounding bone is poor. Insufficient interlock would likely lead to loosening of the cup and require surgical intervention to correct.
One design which relies on this spherical cup interference fit fixation method is that which is described in U.S. Pat. No. 4,892,549 entitled "Dual Radius Acetabular Cup Component" issued to Figgie et. al. and assigned to Osteonics Corporation. The '549 patent describes a dual radius acetabular cup component which is currently marketed by Osteonics Corporation. The dual radius cup relies on an interference fit of the larger spherical surface adjacent to the rim of the cup. The Osteonics design seeks to improve the interference fit by limiting it to the portion of the cup adjacent to the rim. This should provide increased pressure during insertion compared to a cup with one continuous spherical radius. The increased pressure should provide an improved interference fit.
There are two disadvantages to the Osteonics design. First, the design has a rim at an intermediate position on the surface of the cup (transition from the first to the second spherical radius) which provides resistance to insertion. In an acetabulum which has been reamed to the size of the smaller radius, the rim will scrape bone away from the fixation surface during insertion. The removal of bone will lessen the possibility of a strong interference fit.
The second disadvantage of the Osteonics design is that it has a generally smooth outer surface that does not penetrate bone to enhance fixation. The '549 patent describes an affixation-assisting surface treatment, which should provide some frictional resistance to motion at the fixation interface. However, in practice, this surface treatment has been either a porous metal coating or a knurled surface, neither of which provides fixation ability.